Near the beginning of the 20th century it was recognized that a medically useful anatomical image can be obtained when a film containing a radiation-sensitive silver halide emulsion is exposed to X-radiation passing through a patient. Soon thereafter it was recognized that the patient's exposure to X-radiation could be decreased by more than an order of magnitude by placing an intensifying screen adjacent the film. The intensifying screen contains a phosphor that absorbs X-radiation and promptly emits light to expose the film.
Luckey U.S. Pat. No. 3,859,527 introduced storage phosphor imaging. Luckey replaced the prompt emitting phosphor in the intensifying screen with a storage phosphor-one that absorbs X-radiation and stores its energy until subsequently stimulated to emit light imagewise as a function of the X-radiation pattern stored. Thus, the intensifying screen was replaced with a storage phosphor screen (now commonly referred to as an image storage panel), performing both the X-radiation absorption function of an intensifying screen and the image storage function previously provided by the film. This has allowed the film to be eliminated as a required imaging element.
Thereafter, routine testing of known phosphors identified those that are best suited for use as storage phosphors. Subsequent optimizations have led to selection of a storage phosphor family consisting essentially of the product of firing of a combination satisfying the relationship: EQU MFX.sub.1-z I.sub.z.uM.sup.a X.sup.a :yA:eQ:tD (I)
wherein
M is selected from the group consisting of Mg, Ca, Sr, and Ba; PA1 X is selected from the group consisting of Cl and Br; PA1 M.sup.a is selected from the group consisting of Na, K, Rb, and Cs; PA1 X.sup.a is selected from the group consisting of F, Cl, Br, and I; PA1 A is selected from the group consisting of Eu, Ce, Sm, and Tb; PA1 Q is selected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al.sub.2 O.sub.3, La.sub.2 O.sub.3, In.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, GeO.sub.2, SnO.sub.2, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, and ThO.sub.2 ; PA1 D is selected from the group consisting of V, Cr, Mn, Fe, Co, and Ni; PA1 z is from 1.times.10.sup.-4 to 1; PA1 u is from 0 to 1; PA1 y is from 1.times.10.sup.-4 to 0.1; PA1 e is from 0 to 1; and PA1 t is from 0 to 10.sup.-2. PA1 (a) a storage phosphor consisting essentially of the product of firing, at a temperature from about 700.degree. to 1300.degree. C., a combination satisfying the relationship: EQU MFX.sub.1-z I.sub.z.uM.sup.a X.sup.a :yA:eQ:tD (I) PA1 wherein PA1 C.sup.1/2 represents primary or secondary carbon atom, PA1 O represents an oxy linkage, and PA1 Z represents a linking group providing two ring carbon atoms to complete a five-membered ring. PA1 M, F, X, I, M.sup.a, X.sup.a, A, Q, D, z, u, y, e and t are as previously defined; PA1 S' is the oxosulfur reducing agent; and PA1 d is chosen to provide a weight ratio sulfur to alkaline earth (M) of from 0 (preferably 1.times.10.sup.-4) to 2.times.10.sup.-2.
These storage phosphors are hereinafter referred to as Formula I storage phosphors.
A problem that has been encountered in use of Formula I storage phosphors is that released iodine results in reduced levels of transmitted stimulated luminescence, since the blue absorption of iodine captures a significant portion of the stimulated luminescence that would otherwise be transmitted by the phosphor. Phosphor decomposition, resulting in iodine release, has been observed to correlate with the humidity of the environment of use.
It has been proposed to employ in combination with the Formula I storage phosphors a reducing agent for released elemental iodine. The use of oxysulfur reducing agents for iodine are illustrated by Bringley et al U.S. Pat. Nos. 5,427,868, 5,464,568 and 5,507,976.